Just get an old hub carrier w/bearings and axle. Revo ones work good. You attach the wheel for balancing and hold the hub carrier in your hand, spin the tire a bit and it's actually all you need. Smaller than the "prop balancers" and cheaper too. Just make sure it has very free bearings (remove any seals on the bearings for less friction).

And "modeling clay" is what you want. Sold at Target/K-mart etc. Not Play-Dough.